The present invention generally relates to data storage media and devices, and more particularly to data storage devices including bit patterned media.
In conventional magnetic data storage media, data bits are recorded using magnetic transitions on a magnetic recording layer that is composed of a random arrangement of single-domain particles. That is, the magnetic recording layer is typically a thin film of a magnetic material that includes a random arrangement of nanometer-scale grains, each of which behaves as an independent magnetic element. Each recorded bit may be made up of many (50-100) of these random grains.
A stream of data bits is recorded as regions of opposite magnetization on the magnetic recording layer. The boundaries between regions of opposite magnetization occur along the boundaries between the grains. Because the magnetic transitions follow the grain boundaries, the transitions are typically not made along straight lines. Thus, due to the granular nature of the recording layer, the transitions may not be placed exactly where they are intended. Any deviations in grain boundaries represent medium noise, which limits the density of data that can be recorded on the medium.
If the grains are small enough, the magnetic transitions may be straight enough that it is easy to detect which bit cells contain a boundary and which do not. However, if the recording density is increased for a given grain size, the magnetic transitions become proportionally noisier and likely less thermally stable, thereby reducing the ability of the system to accurately recover the data.
An alternative to conventional magnetic recording approaches is to use a bit patterned media (BPM) technique. In bit patterned media, the bits do not contain as many grains as those in conventional media. Instead, bit patterned media comprise arrays of magnetic islands which are defined on a nonmagnetic disk surface during manufacturing. The magnetic islands can be magnetized to a desired polarity one at a time by a magnetic field generated by a write head passing over the islands. The magnetic islands (referred to herein as “dots”) are physically separated from each other by regions of non-magnetic material. These nonmagnetic regions are referred to herein as “gaps” or “spaces.” Thus, the magnetic field generated by a write head in response to a write current can change the magnetization of the dots, while the gaps remain unmagnetized.
Each island, or transition between islands, may represent one bit of information. The signal-to-noise ratio of a bit patterned medium is determined by variations in the spacing and sizing of islands, and can be improved considerably beyond that of conventional media recording schemes.
As the areal density of magnetic disc drives increases, so does the need for more precise control of the location of individual magnetic dots forming tracks, the location of tracks on the disk, the centering of disks relative to their axis of rotation, and the control of head movement while following a designated track.